Fast burst neutronic reactor



y 1968 E. s. BETTIS ETAL. 3,335,759

FAST BURST NEUTRONIC REACTOR Filed May 8, 1967 2 SheetsSheet 1 INVENTOR.

Edward S. Beffis Joseph H.Wesfsik ATTORNEY.

y 1968 E. s. BETTIS ETAL 3,385,759

FAST BURST NEUTRONIC REACTOR Filed May 8, 1967 2 Sheets-Sheet 2 VENTOR.

Edwar Beffis Joseph H. Wesfsrk fl w/4W ATTORNEY.

United States Patent 3,385,759 FAST BURST NEUTRONIC REACTOR Edward S.Bettis, Knoxville, Tenn., and Joseph H. Westsik, Richlaud, Wash.,assignors to the United States of America as represented by the UnitedStates Atomic Energy Commission Filed May 8, 1967, Ser. No. 637,880 8Claims. (Cl. 176--49) ABSTRACT 0F THE DISCLOSURE A pulsed neutronicreactor incorporates a core arrangement wherein molten salt containingfissionable fuel floats on a liquid metal reservoir. The molten saltfuel is driven upward through a tubulation by raising the level of theliquid metal on which it floats. The molten salt fuel becomessupercritical as it passes upward through the tubulation, emits a shortbut intense burst of neutrons, and then becomes subcritical as itcontinues upward. Following the burst, the molten salt fuel passes intoan annular cooling cavity where liquid metal from the reservoir isdischarged into the molten salt to cool it. The molten salt fuel is thenreturned to the region below the core which it occupied prior to itsbeing driven into a supercritical configuration whereupon the aboveprocedure is repeated and another burst obtained.

Background of the invention The invention described herein relatesgenerally to neutronic reactors and more particularly to a molten saltfueled fast burst reactor. It was made in the course of, or under, acontract with the U. S. Atomic Energy Commission.

For certain applications such as materials testing, an extremely highfast neutron flux is required for a very short time period. Pulsed, orfast burst type reactors, are ideally suited for such applicationsinasmuch as they produce high peak neutron fluxes of very short durationand are not limited by the heat-removal requirements of reactorsdesigned to produce high neutron fluxes on a continuous basis. Becausethe burst of neutrons in a pulsed reactor is of such short duration,only a small amount of heat is generated which can be removed from thereactor during the interval between succeeding pulses.

Summary of the invention In accordance with the invention, a molten saltfueled pulsed reactor is provided for generating short intense bursts ofneutrons. The molten salt fuel floats on liquid metal coolant such asmolten lead which is used to raise the molten salt fuel into an activecore region surrounded by neutron reflector. The molten salt becomessupercritical within the active core region, emits a short intense burstof neutrons, and then returns to a subcritical condition. The moltensalt fuel is then displaced from the active core region to a heatexchange region where it is cooled by direct contact with liquid metalcoolant which is discharged into and passes downwardly through themolten salt fuel. The molten salt fuel is then returned to its initialposition below the active core region in preparation for repeating thecycle and generating another burst of neutrons.

Patented May 28, 1968 Brief description of the drawings FIG. 1 showsschematically, in vertical section, a molten-salt-fueled pulsed reactordesigned in accordance with the invention;

FIG. 2 is an enlarged horizontal section taken through the active coreregion of the reactor of FIG. 1; and

FIG. 3 is an enlarged vertical section of the active core region of thereactor of FIG. 1.

Description of the preferred embodiment A pulsed reactor made inaccordance with the invention is illustrated in vertical section inFIG. 1. As shown, the reactor containment vessel comprises an up ersection 1 and a lower section 2 communicating through a centrallypositioned tubulation 3. Upper section 1 provides containment for theactive core region 4- of the reactor and a graphite reflector 5 andcontainer-reflector 6 which enclose and define the outer boundary ofactive core region 4. Lower section 2 of the containment vessel servesprincipally as a reservoir between neutron bursts or pulses by thereactor for the storage of molten salt fuel 7 and liquid metal coolant8. The molten salt fuel, being lighter than and immiscible with theliquid metal coolant, floats thereon. Molten salt fuel 7 preferablycomprises a mixture of lithium fluoride and uranium tetrafluoride, andmolten lead is preferred as liquid metal coolant 8. An annular heatexchange region 9 is defined by the radially outermost wall ofcontainer-reflector 6 and upper section 1 of the reactor containmentvessel. Positioned within heat exchange region 9 are a pair of coolantspray discharge manifolds 10 and 11 for discharging a spray of moltenlead into the molten salt fuel after it has passed through active coreregion 4 and overflowed into heat exchange region 9. A set of turningvanes 12 is disposed immediately above active core region 1 in order todeflect upwardly flowing molten salt fuel discharging from active coreregion 1 radially outward toward heat exchange region 9. Turning vanes12 contain material having a high neutron absorption cross-section inorder to more rapidly terminate the neutron chain-reaction occurring inthe molten salt fuel as it discharges from the active core region 4. Asecond set of plates 13, containing material having a high neutronabsorption cross-section, is positioned in the upper end of tubulation 3immediately below active core region 4. Plates 13 prevent the prematuredevelopment of a chain reaction in molten salt fuel 7 before it entersthe active core region. Braces 14 are provided in active core region 4to support a multiplicity of cups 15 which are described in greaterdetail in reference to FIG. 2 below.

Test materials to be exposed to neutron irradiation are disposed withintest cavity 16 located centrally within active core region 4. Testcavity 16 is provided by a hollow tubular member 17 depending fromflange 18 which provides a closure for upper section 1 of the reactorcontainment vessel.

In operation, a burst of neutrons is obtained by driving a mass ofmolten salt fuel 7 into active core region 4 within graphite reflector 5and container-reflector 6. The molten salt fuel 7 floats on top ofmolten lead coolant 8 which is driven upward through tubulation 3 bymeans of a pulse of pressurized gas injected into region 19 throughconduit 20. The pressurized gas pushes downward on the molten lead inthe annular region between tubulation 3 and lower section 2 of thecontainment vessel, thereby forcing the molten lead and molten salt fuelfloating thereon upward through tubulation 3 into active core region 4.When the molten salt fuel level raises to a position approximately threeinches below the top of graphite reflector 5, the reactor becomescritical on prompt neutrons. As the remainder of the active core regionfills, additional reactivity is introduced to drive the reactor into asuper-critical condition where the neutron flux rapidly increases toprovide the desired pulse or burst of neutrons for irradiating testmaterials within test cavity 16. As the chain reaction progresses, thetemperature of the molten salt fuel increases rapidly causing thereactor to become sub-critical due to the negative temperaturecoefficient of reactivity of the molten salt fuel. The molten lead 8,driven by the pressurized gas pulse in region 10, continues to risethrough the active core region causing the molten salt fuel 7 tooverflow into heat exchange region 9. The entire transit time of themolten salt fuel through active core region 4 is about one-half second.

Heat generated in the molten salt fuel as a result of the fission chainreaction, which occurs as it passes through the active core region 4, isremoved from the molten salt in heat exchange region 9. A pump 21,having its suction connected to the molten lead coolant 8 in lowersection 2 of the reactor containment vessel, pumps relatively coolmolten lead coolant through piping 22 to discharge manifolds 10 and 11which discharge the lead into heat exchange region 9. The salt is thuscooled by direct contact with the lead coolant. The molten salt and leadreach thermal equilibrium rapidly and the heated lead flows through anair-cooled heat exchanger 23 when freeze valve 24 is opened. The flowfrom pump 21 is adjusted to equal the flow through freeze valve 24thereby maintaining the level of molten lead coolant 8 in heat exchangeregion 9 at a constant level.

When the entire mass of molten salt fuel 7 has been cooled to thestarting temperature of about 500 C., the freeze valve 24 is closedwhile pump 21 continues pumping lead into heat exchange region 9. Themolten salt fuel floating on the molten lead in heat exchange region 9is thus displaced upward so as to flow back over the reflector top, andthen downward through active core region 4 to accumulate in tubulation 3below active core region 4 where it floats on molten lead coolant 8.After the entire volume of molten salt fuel is returned to its initialposition below active core region 4, pump 21 is deenergized and freezevalve 24 opened to permit excess molten lead in heat exchange region 9to return to lower section 2 of the reactor containment vessel. Thereactor is then ready for recycling through the above described steps togenerate a new burst or pulse of neutrons.

Referring now to FIGS. 2 and 3, eleven tiers of inverted cups aresupported by braces 14 within active core region 4. The inverted cupshold a total of about one cubic foot of molten salt fuel when completelyfilled. The cups serve the purpose of retaining a portion of theexpanding molten salt fuel and preventing its leaving the active coreregion 4 during the neutron chain reaction so as to partially compensatefor the very large negative temperature coefficient of reactivity of thefuel. Cups 15 may be filled to a variable degree by maintaining thereactor under a vacuum before the molten salt fuel is raised into theactive core region. If the reactor is at atmospheric pressure when theactive core region fills with molten salt, only about one-tenth cubicfoot of molten salt will be retained in the cups prior to the neutronburst so that in effect a multiplicity of voids are created within theactive core region. The higher pressure created during the burst causesthe molten salt to enter the cups reducing the size of the voids thereinand causing a positive reactivity insertion to occur due to the negativevoid coefiicient of reactivity of the reactor. The negative voidcoefiicient of reactivity thus partially compensates for the largenegative temperature coefficient of reactivity. By proper selection ofthe number and size of inverted cups 15 and the pressure within thereactor prior to a burst, the negative temperature coeflicient which isoperative on the burst may be regulated to achieve the desired burstsize.

Technical specifications for a pulsed reactor made according to theinvention substantially as shown in the drawings are listed in the tablebelow.

TABLE Fuel salt composition 73 LiF-27UF (mole percent).

Melting temperature of fuel salt 490 C.

Fuel salt volume l4 ft.

Coolant Lead. Coolant volume ft. Operating temperature range 500 C. to1500 C. Permissible cycles or pulses per hour Burst yield (mw. sec.)1300. Burst yield (neutrons) 10 Burst width or duration 1 millisecond.Core diameter 33 in. Material of construction INOR-8 (coated with Cbwhere contact with molten salt fuel occurs).

The above description of the invention was offered for illustrativepurposes only, and should not be interpreted in a limiting sense. It isintended rather that the invention be limited only by the claimsappended hereto.

What is claimed is:

1. A pulsed neutronic reactor for producing bursts of fast neutronscomprising:

(a) a reservoir of liquid metal;

(b) a vertically oriented tubulation having an upper end and a lowerend, said lower end being immersed in said reservoir of liquid metal;

(c) a mass of fissionable-fuel-bearing molten salt disposed initiallywithin said tubulation and floating on said liquid metal;

(d) neutron reflection means surrounding a portion of said tubulationadjacent its upper end;

(e) means for raising the level of said liquid metal within saidtubulation so as to drive said mass of molten salt upward through saidtubulation, said mass of molten salt becoming super-critical as itpasses upward through said portion of said tubulation surrounded by saidneutron reflection means;

(f) container means surrounding said tubulation and neutron reflectionmeans, said container means and neutron reflection means defining acavity for hold ing said molten salt fuel in a subcritical configurationafter it passes through and discharges from said upper end of saidtubulation;

(g) means for passing liquid metal from said reservoir through saidmolten salt to cool said molten salt; and

(h) means for removing heat from said liquid metal.

2. The neutronic reactor of claim 1 wherein a set of turning vanes isdisposed above said upper end of said tubulation for deflecting saidmolten salt into said cavity as it discharges from said tubulation.

3. The neutronic reactor of claim 2 wherein said turning vanes containmaterial having a high neutron absorption cross-section.

4. The neutronic reactor of claim 1 wherein said liquid metal is lead.

5. The neutronic reactor of claim 1 wherein saidfissionable-fuel-bearing molten salt is composed of a mixture of lithiumfluoride and uranium tetrafluoride.

6. The neutronic reactor of claim 1 wherein compressed gas is used toraise the level of said liquid metal within said tubulation.

7. The neutronic reactor of claim 1 wherein a multiplicity of invertedcups are disposed within said portion of 5 6 said tubulation surroundedby said neutron reflection FOREIGN PATENTS means, said cups providingvoids within said molten salt fuel to partially compensate for thenegative temperature 531797 1/1958 Canada coefiicient of said fuel. 1"

8. The neutronic reactor of claim 1 wherein a test cavity 5 OTHERREFERENCES is provided centrally Within said portion of said tubulationNuclear Science and Engineering, vol. 2, 1957, pp. 797- surrounded bysaid neutron reflection means. 803.

References Cited CARL D. QUARFORTH, Primary Examiner.

UNITED STATES PATENTS 10 H. E. BEHREND, Assistant Examiner. 3,262,8567/1966 Bettis 17649

